1. Field of the Invention
The present invention relates to a driving device for driving a light emitting unit, and more particularly, to a driving device capable of effectively dissipating heat, improving light degradation of the light emitting unit and further extending the effective life of the light emitting unit.
2. Description of the Prior Art
Recently, application fields of light emitting diodes are widely developed. Different from an incandescent lamp, a light emitting diode emits luminescence light, and has advantages of low power consumption, long life, short warming time and rapid response. Moreover, since the light emitting diode is small, shake-proof, easily produced and can be manufactured into a tiny or arrayed element in conformation with different requirements, the light emitting diode is widely applied in an indicator light or a display device in information, communication and consumption electronic products. The light emitting diodes can be applied not only in outdoor monitors and traffic lights but also in any kinds of portable products, e.g. backlight of mobile phones or personal digital assistants, especially liquid crystal displays.
Though the light emitting diode has many advantages, there are some gaps from achieving illumination standards in application, which can be summarized in the following.
First, light degradation. The light degradation means that under the same driving power, luminance of the light emitting diode degrades as operating time increases, and the degradation is most obvious when the light emitting diode is initially activated. In other words, when turning on the light emitting diode at the first time, luminance degradation is most obvious. Therefore, although the light emitting diode has long life, the light degradation and deterioration of fluorescent powder and packaging materials can decrease the luminous flux under long-term operation. When the luminous flux decreases to a specific degree, the effective life of the light emitting diode is over, which cannot achieve requirements of illumination or backlight applications.
Second, heat of a light emitting diode chip. Though volume of the packaged light emitting diode is not small, surface area of the light emitting diode chip in some products is always less than 1 mm2. In such a case, the current density under operation is so large that heat accumulates inside the diode dramatically. If heat cannot be dissipated, the chip is easily damaged. In addition, for achieving some illumination requirements, a lot of light emitting diodes are combined, causing dramatic temperature increase under long-term operation. To dissipate heat effectively, a lot of heat-dissipation areas need to be added, resulting in more difficulties in design of external structures.